Reef Fish Farming Behaviour May Be Promoting Coral Disease

 Anthropogenic driven environmental change has led to the severe degradation of coral reefs worldwide. The occurrence of coral disease has increased significantly in recent years, resulting in mass coral mortality and subsequent habitat loss for countless species. Whilst the exact causes of coral disease continue to elude researchers, well studied coral diseases such as black band disease (BBD) are associated with a consortium of pathogenic microbes infecting the host. As such, gaining a more comprehensive understanding of the processes influencing microbial community structure and disease ecology within coral reef ecosystems is paramount to the efficacy of future conservation efforts.

 Previous research has emphasised the significance of coral-algae interactions in configuring reef microbial communities, yet few studies have addressed the roles that fish may play in mediating these interactions. Consequently, one recent study endeavoured to examine how the grazing activities of territorial damselfish may indirectly alter benthic microbial populations, potentially influencing the prevalence of coral disease.  

 In order to cultivate patches of palatable filamentous algae, damselfish farm the reef benthos, weeding out unfavourable algal species. Accordingly, damselfish engineer reef ecosystems, promoting a large biomass of low-diversity turf algae. However, previous work has illustrated turf algae to be detrimental to coral health, accommodating potentially coral-pathogenic bacteria as well as releasing harmful dissolved compounds. Therefore, it is possible that damselfish farming behaviour could prove damaging to reef health.     

 Casey et al. investigated the benthic microbial communities of shallow reefs surrounding Lizard Island, situated in the northern Great Barrier Reef. Sampling was carried out within the territories of two damselfish species, Stegastes nigricans and Stegastes apicalis, as well as within control plots devoid of territorial grazers. Initially, algal compositions were characterised inside the territories of both species, revealing assemblages dominated by rhodophytes (over 50% coverage from Polysiphonia sp.) with almost all macroalgae eliminated.

 Benthic microbial communities were subsequently characterised by analysing samples of epilithic algal matrix (EAM). EAM is the overriding component of benthos within damselfish territories and is made up largely of turf algae, detritus, and an array of associated microbes. Bacterial assemblages within EAM samples were characterised by 16S rDNA sequencing, revealing that damselfish territories have microbial communities distinct from control plots. EAM microbial communities within S. nigricans and S. apicalis territories, whilst distinct from one another displayed some similarities, probably due to the dominance of Polysiphonia sp. cultivated by both species. Similarly, overlaps were found between EAM microbial communities in S. apicalis territories and control samples. S. apicalis tend to select plots on more flattened areas of benthos, comparable to control plots, likely accounting for these overlaps. In contrast, S. nigricans cultivates algae on the branches of acroporid coral.  

 Analysis of bacterial phylotypes within samples revealed that damselfish territories contained two to three times more potential coral pathogens than control samples. These potential pathogens are cyanobacteria belonging to generas Leptolyngbya and Oscillatoria and have been previously connected to the pathogenicity of BBD. Furthermore, coral disease surveys found that staghorn coral, Acropora muricata, was significantly more likely to be afflicted with BBD within territories of S. nigricans than within control plots. Together, these findings stress the link between damselfish grazing activities altering reef benthos and increased prevalence of microbes associated with coral disease.     

 As damselfish abundance appears to be increasing as an indirect consequence of overfishing, we may expect to see a proliferation in benthos sculpted by their farming behaviours. This study effectively demonstrates the potential adverse consequences for coral reef health associated with such ecosystem alteration, making a valuable contribution to the current understanding of coral reef disease ecology. Nevertheless, research is required to gain a clearer understanding of the mechanisms underpinning such shifts in benthic microbial communities, perhaps also factoring in abiotic variables such as water temperature, known to strongly influence coral disease occurrence.  

Reviewed Paper:

Casey, J. M., Ainsworth, T. D., Choat, J. M. & Connolly, S. R. (2014). Farming behavior of reef fishes increases the prevalence of coral disease associated microbes and black band disease. Proceedings of the Royal Society, 281: 20141032.